It supports a limited number of AM3 CPU's (95W) with the latest BIOS. I would upgrade to either these three:
■Athlon II X3 445
■Phenom II X3 740
■Phenom II X4 945
Since the X4 955 is not a supported CPU, it will either 1) not boot or 2) boot but run as a non-supported CPU at downclocked speeds. You will want to get either of the two I originally listed for your best options. They are supported by the board and will work with the latest BIOS update.